PROJECT SUMMARY/ABSTRACT The toxicity of ultra-potent opioids (fentanyl analogs) is a major factor in the current opioid epidemic. An additional concern with these drugs is that they can be formulated and deployed as chemical weapons, potentially causing mass casualties. Naloxone is the only drug approved for treating opioid poisoning, and while effective in reversing the effects of opioids, including fatal respiratory depression, the utility of naloxone is limited in mass casualty situations because of its short duration of action and its competitive reversible binding at opioid receptors. Even after rescue with naloxone, potentially lethal effects can re-emerge for opioids that have a longer duration of action than naloxone or if additional opioids are administered. Because ultra-potent opioids are toxic when occupying only a small fraction of receptors, reversible antagonists such as naloxone provide only limited, short-term protection. A medication that reverses and protects against the toxic effects of opioids for an extended period and in a manner that is not surmounted by increasing doses of agonist could be particularly effective for counteracting poisoning by ultra-potent opioids. The parent grant of this supplement application (R01DA005018; Discriminative stimulus effects of opioid withdrawal) investigates the abuse-related and toxic effects of opioids in nonhuman primates. The proposed research explores the therapeutic potential of a novel and highly selective opioid receptor antagonist (methocinnamox; MCAM) for treating opioid poisoning. MCAM, an analog of buprenorphine, binds to opioid receptors in a functionally irreversible (i.e., pseudoirreversible) manner, and because it dissociates exceptionally slowly if at all, it is very difficult to displace by opioid receptor agonists. Our pilot data suggest that MCAM has significant advantages over naloxone for rescuing and protecting from opioid poisoning. For example, in monkeys a single injection of MCAM reverses and provides complete protection for several days from a potentially lethal dose of heroin. Moreover, MCAM appears to have an excellent safety profile in vitro and in vivo. The goals of the proposed studies are to develop an autoinjectable formulation of MCAM, possibly including a sustained-release formulation, with physiochemical properties consistent with deployment as an antidote for opioid poisoning (Aim 1), and then to characterize the ability of those formulations to reverse and protect against the respiratory depressant effects of ultra-potent opioids (fentanyl and the fentanyl analog carfentanil; Aim 2). Aim 2 will also compare antagonism by MCAM to its detection in monkey plasma. We hypothesize that, in contrast to the short-duration, surmountable antagonism provided by naloxone, autoinjectable formulations of MCAM would be suitable as antidotes for opioid poisoning in the event that fentanyl analogs are used as chemical weapons, and will provide sustained, insurmountable antagonism of the toxic effects of ultra-potent opioids. These studies test proof-of-principle of using an autoinjectable formulation of a highly selective, potent, pseudoirreversible opioid receptor antagonist as an antidote to opioid poisoning.